The United States' projected annual energy growth from 2010 through 2035 is 0.3%, from which 37% will be produced from petroleum resources (Annual Energy Outlook, U.S. Energy Information Administration, Technical Report, 2012). Chemical flooding, an enhanced oil recovery (EOR) method, has been used to improve oil production from conventional reservoirs (D. O. Shah, and R. S. Schechter, Improved oil recovery by surfactant and polymer flooding, Academic press, Inc., New York, 1977). Surfactants have been used as EOR agents to decrease interfacial tension (IFT) between oil and brine, leading to an increase in surface area of the oil droplets (smaller droplets) and higher oil production (J. Eastoe, Advanced surfactants and interfaces, Bristol UK, 2003).
Oil production from ultra-tight reservoirs is rapidly growing and will dominate in the next 10-15 years in the United States (AEO 2013 Early Release Overview, U.S. Energy Information Administration, Technical Report, 2013). However, ultra-tight reservoirs have a complicated mineralogy including clay minerals and clastic minerals like quartz, feldspar, and calcite. Moreover, ultra-tight reservoir rocks have interstitial pore sizes ranging from 1-3 nm to 400-750 nm (C. Zou, Unconventional Petroleum Geology, Newnes, 2013). The small interstitial pore sizes lead to high capillary pressures, which act as a barrier to fluid mobility in ultra-tight reservoirs. Horizontal drilling and hydraulic fracturing processes to enhance the reservoir permeability overcome the barrier only partially.
In hydraulic fracturing, reservoir rock is cracked by pumping fluids into the wellbore and rock formation. Fracturing fluids comprise municipal water, proppant, and chemical additives such as surfactants (C. Clarck, A. Burnham, C. Harto, and R. Homer, Hydraulic fracturing and shale gas production: Technology, Impacts, and Regulations, Argonne National Laboratory, Technical Report, 2013). Surfactants are often added to fracturing fluids to enhance their imbibition into reservoir rock.
Safety concerns have forced a shift toward environmentally friendly surfactants, such as polyoxyethylenated (POE) straight-chain alcohols R(OC2H4)xOH. POEs are biodegradable and, compared to other nonionic surfactants, have greater tolerance to high ionic strength and hard water conditions (M. J. Rosen and J. T. Kunjappu, Surfactants and Interfacial Phenomena, Wiley, 2012).
The behavior of POE surfactants changes as POE structures are altered. The differences in POE surfactant behavior can be measured through surface/interfacial tension (IFT), emulsification, solubilization, and turbidity of surfactant solutions (Tharwat F. Tadros, Applied Surfactants: Principles and Applications, Wiley-VCH, 2005).
The decline in IFT using surfactants enhances the dispersion of one phase in another, resulting in the formation of emulsions (S. Kokal, Crude oil emulsions: A state-of-the-art review, SPE Production & Facilities, vol. 20, no. 1, pp. 5-13, 2005). Emulsions and microemulsions may cause formation damage, particularly in tight reservoirs with low porosity and permeability. They may also cause pressure drops in flow lines and the production of off-spec crude oil (S. Kokal, Crude oil emulsions: A state-of-the-art review, SPE Production & Facilities, vol. 20, no. 1, pp. 5-13, 2005). Thus, demulsifying surfactants are used in petroleum reservoirs to avoid operational difficulties during production.
Microemulsion formation depends on a surfactant's surface activity (M. J. Rosen and J. T. Kunjappu, Surfactants and Interfacial Phenomena, Wiley, 2012). As surfactant molecules diffuse from the bulk phase to the oil/brine interface, the surfactants' hydrophobic tails adsorb on the oil phase and their hydrophilic heads partition into the aqueous (brine) phase. Interface partitioning of surfactants increases with increasing hydrophilic chain length, thus increasing the probability of microemulsion formation. For a surfactant to behave as an emulsifier, large surface activity coupled with low IFT values is generally required. This is achieved by increasing the length of the hydrophobic chain (K. Shinoda, H. Saito, H. Arai, Effect of the size and the distribution of the oxyethylene chain lengths of nonionic emulsifiers on the stability of emulsions, Journal of Colloid and Interface Science, vol. 35, no. 4, pp. 624-630, 1971). However, although lowering IFT enhances emulsion stability, ultra-low IFT can destabilize the emulsions (P. D. Berger, C. Hsu, and J. P. Arendell, Designing and selecting demulsifiers for optimum field performance on the basis of production fluid characteristics, SPE Production Engineering, vol. 3, no. 4, pp. 522-526, 1988; H. L. Rosano and D. Jon, Considerations on formation and stability of oil/water dispersed systems, Journal of the American Oil Chemists' Society, vol. 59, no. 8, 1982; and Y. Yang, K. I. Dismuke and G. S. Penny, Lab and field study of microemulsion-based crude oil demulsifier for well completions, SPE International Symposium on Oilfield Chemistry, Texas, USA, April 2009).
Demulsifying efficiency has been shown to result from equal partitioning of surfactants between oil and brine phases (M. A. Kelland, Production chemicals for the oil and gas industry, CRS Press, 2009; M. A. Krawczyk, D. T. Wasan, and C. S. Shetty, Chemical demulsification of petroleum emulsions using oil-soluble demulsifiers, Industrial & Engineering Chemistry Research, vol. 30, no. 2, pp. 367-375, 1991; P. D. Berger, C. Hsu, and J. P. Arendell, Designing and selecting demulsifiers for optimum field performance on the basis of production fluid characteristics, SPE Production Engineering, vol. 3, no. 4, pp. 522-526, 1988). However, weak emulsifiers that are also IFT reducers may also be beneficial to oil recovery (L. Xu, Q. Fu, Methods for selection of surfactants in well stimulation, US 2013/0067999 A1, 2013). Therefore, to generate less emulsion and high oil recovery in a surfactant/rock/oil/brine system, it may be desirable to use surfactants with none or weak emulsifying ability and low IFT (L. Xu, Q. Fu, Methods for selection of surfactants in well stimulation, US 2013/0067999 A1, 2013).
Surfactant solubility indicates a surfactant's ability to remain active in brine and to travel into the rock matrix at reservoir temperature. Further, surfactant solubility in aqueous solution may directly impact oil recovery. The solubility of an aqueous nonionic surfactant solution depends on temperature and is manifested by a cloud point temperature (CPT).
CPT greatly depends on the arrangement of hydrophobic and hydrophilic parts of surfactants. Cloud point studies of several polyoxyethylene (POE)-type nonionic surfactant solutions in 1-butyl-3-methylimidazolium tetrafluoroborate suggest that CPT increases with increasing POE chain length and decreases with increasing hydrocarbon chain length (T. Inoue and T. Misono, Cloud point phenomena for POE-type nonionic surfactants in a model room temperature ionic liquid, Journal of Colloid and Interface science, vol. 326, no. 2, pp. 483-489, 2008).
The impact of surfactants on parameters like surface/interfacial tension, contact angle, solubility, and emulsification have been performed at ambient conditions for applications in oil recovery from conventional and tight reservoirs (K. Makhanov and H. Dehghanpour, An experimental study of spontaneous imbibition in Horn River shales, SPE Canadian Unconventional Resources Conference, Calgary, Alberta, Canada, 2012; A. Bera, K. Ojha, A. Mandal, and T. Kumar, Interfacial tension and phase behavior of surfactant-brine-oil system, Colloids and Surfaces A, vol. 383, pp. 114-119, 2011; A. Bera, A. Mandal, and B. B. Guha, Synergistic Effect of Surfactant and Salt Mixture on Interfacial Tension Reduction between Crude Oil and Water in Enhanced Oil Recovery, Journal of Chemical Engineering Data, vol. 59, no. 1, pp. 89-96, 2013; A. Seethepalli, B. Adibhatla, K. K. Mohanty, Wettability alteration during surfactant flooding of carbonate reservoirs, Journal of Chemical Engineering Data, SPE/DOE Symposium on Improved Oil Recovery, Tulsa, Okla., April, 2004; A. S. Zelenev, CESI Chemical, a Flotek Industries Company, Surface energy of North American Shales and its role in interaction of shale with surfactants and microemulsions, SPE International Symposium on Oilfield Chemistry, Texas, USA, 2011; A. S. Zelenev, L. M. Champagne, M. Hamilton, Investigation of interactions of diluted microemulsions with shale rock and sand by adsorption and wettability measurements, Colloids and Surfaces A, vol. 391, no. 1-3, pp. 201-207, 2011). However, these parameters have rarely been examined at reservoir conditions.
Further, although the adsorption of surfactants at liquid/liquid or liquid/solid interfaces is a dynamic process, surface/interfacial tension, contact angle, solubility, and emulsification have been primarily studied at equilibrium conditions (D. Nguyen, D. Wang, A. Oladapo, J. Zhang, J. Sickorez, R. Butler, and B. Mueller, Evaluation of Surfactants for Oil Recovery Potential in Shale Reservoirs, SPE Improved Oil Recovery Symposium, Tulsa, Okla., USA, April 2014). In addition, very limited comprehensive studies exist to systematically screen surfactant structure for enhanced oil recovery, particularly for tight reservoirs.